This invention relates to an electric motor drive control apparatus suitable for use with a vehicle power steering unit.
For example, Japanese Patent Kokai No. 2-81769 discloses an electric motor drive control apparatus which employs four switching transistors for pulse width modulation (PWM) control of an electric motor used in assisting the driver's vehicle steering operation. When the current flow through the electric motor exceeds a first predetermined value, the transistors are controlled to interrupt the power to the electric motor. The transistors are returned to the initial states to resume the PWM control of the electric motor when the motor current decreases below a second predetermined value less than the first predetermined value. With such a conventional electric motor drive control apparatus, the transistors are switched repetitively between two states every time the motor current exceeds the first predetermined value. If the transistors are switched at a very short repetitive period, there will be great transient power losses to generate an excessive amount of heat from the transistors so as to degrade the operation reliability.